Let her go and stretch her wings.
DD1 went to her first festival last year, aged just 17 (end of Year 12). As someone else said, it's a rite of passage. All of her huge group of friends camped together and partied together and she told me she felt entirely safe the whole time - everyone was so friendly.
She could put up with the messiness/ dirtiness/ foul loos/ crap food, excess alcohol, drunk people staggering around day and night for three days but was very, very relieved to get home to a hot bath and clean bed. This is all part of it... I sensed that, as she went off, she was desperately wanting to prove to herself and the world that she is a grown-up, independent young adult and, sure enough, she made a success of the experience. However she was VERY happy to flee back home again to a clean and cosy house and Mum's healthy home cooking after three days of eating and living amongst rubbish.
I agree with others:
DD got a cheap-as-chips tent from Tescos and left it there.
Definitely don't take a good phone - dig out a crappy old handset
Nearer the time, post again asking what she should take and you'll get all the info you need.
The other thing that DD said was a definite lesson for her for next time was to be picked up to go home at the end of the last night, on a high, after the last performance. She said that by then she had had enough and was ready to leave. Waking up the next morning surrounded by filth and mud and litter and wasted people was the low spot of the whole experience. She said the whole site just looked like landfill. Quite how the logistics of that would work, I don't know - I guess they'd have to pack up all their stuff into bags in the morning and leave everything in the tent ready to collect and then be picked up.
DD is really glad she went because it was an experience/ rite of passage and she would have felt she was missing out if all her friends had done it without her.
This year she only wants to go for the day though!